This invention relates to polymeric compositions made from multi-functional monomers, especially multi-functional acrylate monomers such as 1,4-butanediol diacrylate and/or 1,6-hexanediol diacrylate, a process to make the polymeric compositions, and the use of the polymeric compositions as spacers in liquid crystal displays. Other embodiments of the invention relate to providing the spacer particles with adhesive properties or conductive properties or further coloring the spacer particles.
It is very important to have precise control of the thickness of the liquid-crystal layer in liquid-crystal-based displays. The liquid-crystal layer acts as an electro-optic light valve that works in conjunction with polarizers to modulate the transmission of light through a display between two states: one of off, where the liquid crystals block substantially all light; and one of on, where the liquid crystals allow transmission of light. Irregularities in the thickness of the liquid-crystal layer, also known as the cell gap, result in uneven display performance affecting such properties as contrast, transmittance, and the response time of the liquid-crystal layer to an electric signal.
Liquid crystal displays have a structure such that two substrates, generally glass or plastic sheets, are disposed opposite to each other through optionally a color filter on the inside surface of the top substrate (top is the side toward the viewer), an alignment layer, an electrode layer, a spacer particle, and a liquid crystal layer. A purpose of the spacer particle is to control the thickness of the liquid crystal layer and another purpose is to provide a uniform thickness of the liquid crystal layer over the entire active area of the display. Other means for controlling the cell gap include the flatness of the substrate material, the flatness of the layers between the substrates, the number of spacer particles in any given area, and the spatial distribution of spacer particles ) with respect to one another.
Particles generally suitable for use as spacers in liquid crystal displays are chosen from among: glass; oxides of silica, alumina or other ceramics; and plastics. The shape of particles generally suitable for use as spacers in liquid crystal displays are chosen from among: cylindrical rods having aspect ratios from about 1:2 to greater than 1:10; and spherical balls. The choice of spacer particles is largely dictated by the characteristics of the spacer particles that include but are not limited to: uniformity of the particle diameter, amount of impurities that may leach into the liquid crystal layer, compatibility with the liquid crystal layer, hardness, compressibility, coefficient of thermal expansion, elastic modulus, refractive index, thermal stability, and dielectric constant.
The importance of spacer particles in maintaining the cell gap has been described herein. In addition, spacer particles are very important in establishing the correct cell gap during the assembly of liquid crystal displays. The liquid-crystal display assembly process generally requires the following steps: a) spacer particles are deposited in a pre-determined concentration onto one sheet of glass or plastic substrate, b) a sealant is applied along the edge of the same substrate in a fashion similar to a picture frame leaving a small gap that will later be used for filling with the liquid crystal material, c) a second sheet of glass or plastic substrate is placed over the first substrate containing the spacer particles and the adhesive, d) the two substrates are pressed together at an elevated temperature to cure the adhesive and therefore sealing the substrates together. The properties of the spacer particles must be such that the particles do not degrade during the application of heat and pressure in the sealing process; the spacer particles must have sufficient thermal stability to withstand heating and also good compression strength so as to not break or fracture under load.
Plastic spacers will deform in the edge-sealing process described above. The extent of deformation can vary significantly, and is a function of the composition of the spacer, the amount of pressure applied during the sealing process, and the heat applied during the sealing process. It is preferred that a plastic spacer deform slightly when exposed to heat and pressure and then recover some or all of its original shape when the heat and pressure are removed. The extent of recovery, or alternatively, the extent to which the spacer particle is deformed and then resumes some or all or its original diameter, is known as the recovery factor. The recovery factor is described in detail in W. O. Patent 9206402, see in particular FIG. 4 of the cited patent application. In the measurement of recovery factor a given load is applied to a spacer particle and the displacement of the spacer caused by the load is measured (L.sub.1). The load is then removed and the extent to which the original particle diameter recovers is given as (L.sub.2). The recovery factor is calculated by (L.sub.2 /L.sub.1). In view of the variations in display quality, there is a continuing need for spacer particles that have a balance of properties affecting control of the cell gap during, and after, the cell sealing process and impacting both the thermal stability and the recovery factor of the spacer.
W.O. Pat. Appl. No. 9206402 discloses spheres with certain elastic modulus and recovery factor properties. The spheres may be applied as spacers for liquid-crystal display elements. The spheres are made of polydivinylbenzene, divinylbenzene-styrene copolymer, divinylbenzene-acrylate copolymers, or polydiallylphthalate.
U.S. Pat. No. 5,231,527 discloses a liquid crystal display with two sheets of substrates disposed opposite to each other, transparent electrodes, orientation films, a spacer particle with a certain range of elastic modulus, and a liquid crystal layer. The spacer particle may be made of a crosslinked vinyl copolymer.
Despite the teachings of the disclosures, there is a continuing need to provide improved spacer particles of uniform size which, and which possess, desirable physical characteristics.